There are two conventional methods of manufacturing optical filters and other thin-film-coated substrates based on techniques such as vacuum evaporation and sputtering. One comprises the measurement of the thickness of the thin film produced and control of the film coating process based on the results of such measurement, while the other provides no such control, and both are widely used.
The method that does not incorporate thin film thickness measurement aims to achieve the desired thickness by, for example, controlling the film formation time while keeping the conditions of the film formation atmosphere and speed constant. In the case of producing a thin film while moving the substrates to be coated, the desired film thickness may be obtained by controlling their moving speed.
With such a thin-film-coated substrate manufacturing method, it is not possible to check whether the thin film being produced would satisfy the required specification until the film coating process is completed, as the film thickness is not measured during the process. A thin film production apparatus normally operates in a vacuum so that it is not desirable to frequently take out the substrate on which a thin film is being produced to measure its thickness. Therefore, where more than one thin film is generated, measurement is not carried out until the production of all thin films is complete.
However, operating conditions of thin film production apparatus, such as film formation speed, are generally susceptible to fluctuations, and these have quite frequently caused the film thickness to fluctuate, resulting in the production of coatings that do not meet the specifications. Therefore, with thin film production methods in which film thickness measurement was not used, it was difficult to sustain the yield above a certain level.
As a result, the control of the film coating process through the measurement of film thickness began to be practiced. Taking the production of anti-reflection coatings and other optical coatings over plane substrates, such as glass plates, as an example, conventional methods of manufacturing thin-film-coated substrates in which the film thickness is measured and the film coating process is controlled based on the results of such measurements are explained with reference to the accompanying drawings.
FIG. 7 is a schematic diagram which shows the thin-film-coated substrate manufacturing process as viewed in a direction parallel to the substrate surface and perpendicular to the direction of the movement of the substrates. Inside a vacuum vessel 16, a thin film material 3 is placed, with the film-forming particle flux generating element 2 pointing upwards in the diagram. Electrons are ejected from an electron gun 6, and travel to the film-forming particle flux generating element 2 due to the effect of a magnetic field, not shown on the diagram. On collision with the element, they heat it, thereby generating a film-forming particle flux 5. The film-forming particle flux axis 9, i.e. the axis of the film-forming particle flux 5 (representing the direction in which film-forming particles are generated in the highest density), points upwards in the diagram. A film formation monitoring plate 8 is placed on the film-forming particle flux axis 9 or in its vicinity, and a film thickness measuring device 7, which optically measures the thickness of the thin film formed on the film formation monitoring plate 8, is located above it. An arrangement is made so that a thin film is formed on the underside of each of the group of substrates to be coated 1 through exposure to the film-forming particle flux 5 in the vicinity of the film-forming particle flux axis 9, as they move along a path set between the film-forming particle flux generating element 2 and the film formation monitoring plate 8 in a direction perpendicular to the film-forming particle flux axis 9 (from left to right on the diagram). Also, to limit the range of the group of substrates to be coated 1 exposed to the film-forming particle flux 5 along their movement path, corrector plates 4 are placed between the film-forming particle flux generating element 2 and the group of substrates to be coated 1. A shutter 11 that shuts off the film-forming particle flux 5 as necessary is also provided.
FIG. 8 views this set-up from the location of the film-forming particle beam generating element 2 along the film-forming particle beam axis 9. The group of substrates to be coated 1 is arranged in, for example, two columns as shown in the diagram, ensuring that a gap is created so that the film-forming particle beam 5 is not blocked along the film-forming particle beam axis 9 and in its vicinity. The film formation monitoring plate 8 is placed in such a way that film-forming particles reach it through this gap.
When producing a thin film on each of the group of substrates to be coated 1, the film-forming particle beam generating element 2 of the thin film material 3 is continuously heated with the electron beam generated by the electron gun 6, causing it to emit film-forming particles. Initially, the shutter 11 is closed, and film-forming particles cannot reach the substrates to be coated. When the temperature of the film-forming particle beam generating element 2 reaches a steady state, the rate of film-forming particle generation also reaches a steady state. Upon confirming this, the shutter 11 is opened and at the same time the group of substrates to be coated 1 is made to move at a constant speed from left to right on FIG. 7. The film-forming particles generated by the film-forming particle beam generating element 2 travel along the film-forming particle beam axis 9, spreading outwards as the traveling distance increases, and reach the group of substrates to be coated 1. Of the space occupied by film-forming particles, the region in which film-forming particles are in sufficiently high concentration is defined as the film-forming particle beam 5. Some film-forming particles, traveling along the film-forming particle beam axis 9 or in its vicinity, reach the film formation monitoring plate 8 through the gap created between substrates.
The thickness of the thin film on each of the group of substrates to be coated 1 is measured indirectly by measuring the thickness of the monitor thin film formed on the film formation monitoring plate 8 using the film thickness measuring device 7. The formation of a thin film on the film formation monitoring plate 8 occurs under similar conditions to those for each substrate in the group of substrates to be coated 1. Therefore, the thickness of the monitor thin film has a certain relationship with that of the thin film formed on each substrate in the group of substrates to be coated 1. Ideally, when thin films are formed on 10 groups of substrates to be coated 1, the thickness of the monitor thin film formed on the film formation monitoring plate 8 becomes approximately 10 times that of the thin film formed on one group of substrates to be coated 1. In practice, such a correspondence is accurately determined through experiments. From this result, the correspondence between the time rate of change in the thickness of the monitor thin film formed on the film formation monitoring plate 8 and the time rate of change in the thin film formed on the groups of substrates to be coated 1 can also be obtained.
Based on these thin film thickness measurement results, the film coating process is controlled, involving, for example, the adjustment of the film formation speed or physical properties of the thin film, such as refractive index, and the closing of the shutter 11 as soon as the desired film thickness is obtained.
Methods of controlling the film coating process through the measurement of the thickness of a thin film obtained along the film-forming particle beam axis or in its vicinity through the use of a film formation monitor as shown above have been disclosed, for example, in JP-A-01-306560.
However, the inventors of the present invention discovered that such conventional thin-film-coated substrate manufacturing methods had problems as shown below.
Namely, to have a monitor thin film formed on the film formation monitor by placing it on the film-forming particle beam axis or in its vicinity for the purpose of thin film thickness measurement, it was necessary to arrange or move the substrates to be coated so as to prevent them from blocking film-forming particles and create a gap around the center of the film-forming particle beam. Nevertheless, this hardly became an issue, as far as small substrates to be coated, such as optical lenses, were involved, as they were placed in a large number of columns.
However, the inventors of the present invention found that placing a film formation monitor on the film-forming particle beam axis or in its vicinity for thin film thickness measurement led to an undesirable over-sizing of the production facility or a sharp fall in productivity, when producing a thin film on a large substrate such as the surface anti-reflection filter of a display device with a diagonal size of 14 in. (35 cm) or more. Namely, to prevent substrates to be coated from blocking the film-forming particles around the center of the film-forming particle beam, the group of substrates to be coated 1 had to be arranged in at least two columns as shown in FIG. 8 to ensure that a gap was always created between substrates to be coated. Therefore, the area in which the group of substrates to be coated 1 were exposed to the film-forming particle beam 5 (film formation area 20 as shown with a two-dot chain line in FIG. 8) needed to be more than twice the width of the substrate to be coated.
Furthermore, when fixed film formation monitors were placed in the vicinity of the film-forming particle beam axis, it was sometimes impossible to control the film coating process by means of a film formation monitor, as the substrates to be coated or their supports came in the way, lying between the film formation monitor and film-forming particle beam generating element, as the dimensions of the substrate to be coated changed due to a change in the type of substrate to be coated to be manufactured. In other words, controlling a film coating process using fixed film formation monitors required severe restrictions to be placed on the dimensions or shapes of substrates to be coated.
Moreover, as far as the inventors of the present invention know, there are cases where the thickness of the monitor thin film formed on the film formation monitor becomes far thinner than that of the thin film formed on the film formation monitor, if a fairly large gap between substrate columns is not secured. The inventors of the present invention have discovered that this is mainly due to the fact that some of the traveling film-forming particles become deflected due to collision with the remaining gas molecules in the vacuum vessel and caught by the substrates to be coated or substrate holders that support them. FIG. 9 shows this mechanism. If there were no gas molecules inside the vacuum vessel, such film-forming particles would travel from the film-forming particle beam generating element to the film formation monitor through the gap between substrate columns without any interference. In such a case, therefore, the thickness of the monitor thin film formed on the film formation monitor would become almost the same as that of the thin film formed on substrates to be coated. In reality, however, some gas molecules always remain in the vacuum vessel due to a limitation in the performance of the vacuum pump or the necessity of the film coating process. As a result, film-forming particles reach the substrates to be coated or film formation monitor after colliding with these gas molecules. Since there is no obstacle between the film-forming particle beam generating element and substrates to be coated, those film-forming particles having collided with gas molecules can still reach substrates to be coated, although their traveling paths are zigzagged. In the case of the film formation monitor, however, the film-forming particles having collided with gas molecules become blocked by substrates to be coated etc., unless the gap between substrate columns is made quite large, as the substrates to be coated are placed before it with respect to the film-forming particle beam generating element. Also, the distance from the film-forming particle beam generating element is generally greater for the film formation monitor than substrates to be coated, when it is placed in the vicinity of the film-forming particle beam axis. Therefore, an increasing proportion of the film-forming particles fail to reach the film formation monitor due to collision with gas molecules though they are roughly in the direction towards the film formation monitor. As a result, the thickness of the thin film formed on the film formation monitor becomes, for example, less than half that of the thin film formed on the substrates to be coated. Therefore, it was necessary that the width of the film formation area be set considerably greater than twice the width of the substrate to be coated.
For example, to form a thin film on the surface anti-reflection filter (shorter side 31 cm.times.longer side 38 cm) of a display device with a diagonal size of 17 in. (43 cm), the width of the film formation area was required to be considerably greater than 62 cm, which was twice the length of the shorter side, (e.g. 70 cm or more). The seriousness of this problem increases as the size of the substrate to be coated increases.
Even when the width of the film formation area is quite large, severe restrictions are unavoidably placed on the dimensions or arrangement of substrates to be coated that form a substrate group (e.g. whether to set the moving direction of the substrates along its longer side or perpendicular to it), to ensure that a gap is created in the middle of the film formation area. Therefore, even where there was no obstacle to obtaining a wide film formation area from the production facility point of view, in many cases it was not possible to take full advantage of it due to restrictions in terms of sizes and arrangements of substrates to be coated, leading to a severe loss in productivity. For example, when a thin film is produced on the surface anti-reflection filter (shorter side 31 cm) of a display device with a diagonal size of 17 in. (43 cm) using a production facility with a 1 m-wide film formation area, it is physically possible to place such substrates to be coated in three columns. Nevertheless, it was necessary to limit the number of columns to two due to the need to arrange the substrates to be coated in such a way that they would not block the film formation monitor placed in the vicinity of the film-forming particle beam axis (i.e. around the center of the film formation area). Namely, it was only possible to manufacture the above type of filters at 2/3 of the potential productivity.
Also, since it is common to produce various types of thin-film-coated substrates using the same production facility, placing the film formation monitor on the basis of a particular type of substrate to be coated and its specific arrangement pattern unavoidably gives rise to the same productivity problem as the one above, when making other types of products.
Moreover, when the substrate to be coated is a tape-like material made of a plastic etc., thin film production is sometimes carried out continuously, as the substrate to be coated is unwound from the substrate roll. In such cases, it was necessary to, for example, limit the width of each of the parallel tape-like substrates to less than 1/2 that of the film formation area and unwind it after winding it onto a pair of substrate rolls. In such cases, an undesirable over-sizing of the production facility or reduction in productivity due to constraints on the width of the tape-like substrate was unavoidable for the same reason.
The following seem to be the reasons why thin film thickness measurement is carried out by placing a film formation monitor on the film-forming particle beam axis or in its vicinity, despite the accompanying undesirable increase in the size of the production facility or sharp drop in productivity:
Firstly, the production of a thin film on large substrates to be coated was not undertaken until recently, and problems such as restrictions on the arrangement of the substrates during film production etc., as mentioned above did not come to the fore. Secondly, a thin film formed by film-forming particles traveling along the film-forming particle beam axis or in its vicinity is believed to be most suitable for the monitoring of the film coating process as its characteristics are superior to those of a thin film formed by film-forming particles moving away from the film-forming particle beam axis. This is because film-forming particles traveling along the film-forming particle beam axis or in its vicinity are more numerous and generally have greater kinetic energy, so that they can form homogenous thin films with greater refractive index. In contrast, film-forming particles moving away from the film-forming particle beam axis are relatively few and have lower energy, so that thin films formed of them are low in refractive index and thinner.
Therefore, the placement of the film formation monitor in the vicinity of the film-forming particle beam axis has been treated as a foregone conclusion and other methods have not been considered.
The object of the present invention, therefore, is to offer a thin-film-coated substrate manufacturing method and apparatus that makes it possible to produce multi-type (size) products without the undesirable over-sizing of the production facility or productivity setbacks associated with increases in substrate size and adoption of continuous (tape-like) substrate production.